Why African Slaves?
In the late 15th century, Europeans (Spanish and Portuguese first) began to explore, colonize and conquer the territory in the Americas. The European colonists attempted to enslave some of the Native Americans to perform hard physical labor, but found them unaccustomed to hard agrarian labor and so familiar with the local environment that it was difficult to prevent their escape. Their lack of resistance to common European diseases was another factor against their suitability for slavery. The Europeans had also noted the West African practice of enslaving prisoners of war (a common phenomenon among many peoples on all of the continents). European colonial powers traded guns, brandy and other goods for these slaves, but this had little effect on the Arabian and African trade. The African slaves proved more resistant to European diseases than indigenous Americans, familiar with a tropical climate and accustomed to agricultural work. As a result, regular trade was soon established. Source